The invention relates to a stock-inlet or head box for a papermaking machine. Among the essential components of this stock-inlet are two converging flow-guidance walls which are as wide as the machine and which, together with a rear wall and with two lateral walls, define a nozzle-like outlet-channel as wide as the machine. Located at the downstream end of the flow-guidance walls, which are also known as the lower lip and the upper lip, is an outlet-gap as wide as the machine, from which the paper-stock emerges, in the form of a jet as wide as the machine, onto a circulating wire-screen upon which the paper web is formed.
Known stock-inlets of this kind are described in the following publications:
1. C.A.-PS 849,817,
2. U.S. Pat. No. 3,373,080,
3. U.S. Pat. No. 4,198,270 (=DE 27 26 709),
4. U.S. Pat. No. 4,455,197,
5. U.S. Pat. No. 4,552,619,
6. U.S. Pat. No. 2,920,699,
7. U.S. Pat. No. 3,846,229 (=DE 23 02 196)
One serious problem with such stock-inlets is maintaining the internal width of the above-mentioned outlet-gap constant over the width of the machine. It has been found, in practice, that the presence of local deviations from the desired width of the gap impairs the quality of the resulting strip of paper. More particularly, it produces an irregular weight-per-unit-of-area transverse profile. For instance, it has been found that a specific change in gap-width increases the weight per unit of area of the strip of paper by a factor of 10.
Another problem is that certain changes in gap-width may occur only when the machine is in operation and thereafter disappear only partly. Such changes are brought about by temperature fluctuations, for example, especially if, after a shut-down, the papermaking machine is started up again with heated stock. In this case, the flow-guidance walls assume only gradually the higher temperature of the stock.
Changes in gap-width which occur in different degrees at different locations over the width of the machine are particularly troublesome. For instance, a flow of cold air entering the mill (such as, when a door is opened) may cause a one-sided change in gap-width. Other uneven changes in gap-width appear to be caused by stiffening ribs which are used, in certain known designs, to reinforce the flow-guidance walls.
In Publication 1, a description is given of a stock-inlet, the lower flow-guidance wall of which is supported on a foundation by a hollow carrier. The problem upon which this publication is based is that the upper part of the hollow carrier, adjoining the flow-guidance wall, assumes a higher temperature than the lower part. The top, therefore, expands more than the bottom and the lower flow-guidance wall, therefore, becomes warped. In-order to overcome this problem, it is suggested that the lower part of the hollow carrier be kept at the same temperature as the upper part by means of a heating device.
In the case of the stock-inlet according to Publication 4, a bundle of pipes, and flow-guidance walls attached thereto, runs through the interior of a hollow stock-inlet housing. A pivotably mounted upper lip is formed by one of the walls of a cross-sectionally triangular hollow carrier. In order to keep the parts of the stock inlet at a specific temperature, water is passed through the interior of the stock-inlet housing and through the upper-lip hollow carrier, separate from the paper-stock, of course. According to FIG. 6, a rectangular hollow carrier is built onto the triangular hollow carrier, which, according to FIG. 1 serves to support the adjusting spindles. In order to ensure that, in the event of a change in temperature a corresponding change in length of the triangular hollow carrier can take place unimpededly, rectangular hollow carrier 54 in FIG. 7 is divided into sections distributed over the width of the machine.
In the stock-inlet described in Publication 5, the flow-guidance walls are again reinforced by hollow carriers. As in Publication 1, steps are taken to ensure that the hollow carrier, and thus the flow-guidance walls, do not become warped by differences in temperature.
None of these known arrangements has produced satisfactory results. On the one hand, stiffening the flow-guidance walls with hollow carriers leads to very costly and bulky designs. On the other hand, the devices used to maintain uniform temperatures do not react fast enough, at least when the papermaking machine is started up. This means that, when the machine is started up, there is still a danger of the flow-guidance walls expanding faster than the opposing wall of the hollow carrier. There is also frequently a danger of at least one of the two flow-guidance walls being temporarily unevenly heated. For instance, it is possible for the inside of a flow-guidance wall in contact with the paper-stock to be at a higher temperature than the outside. This may also cause the relevant flow-guidance wall to arch inwardly in the central part of the width of the papermaking machine, thus reducing the internal width of the outlet-gap. This danger may be even greater if, as in Publication 3, the flow-guidance walls are in the form of simple, self-supporting plates with no stiffening hollow carriers. In the simplest case, the two converging flow-guidance walls are secured rigidly to a carrier-element extending over the width of the machine, for example to a rear wall of the outlet-channel in which a bundle of tubes which feed the paper-stock is arranged.